The same fact is seen in geological distribution. Mr. Wallace observes: Most of the larger and some smaller groups extend through several geological periods. In each period, however, there are peculiar groups, found nowhere else, and extending through one or several formations. As generally in geography no species or genus occurs in two very distant localities without being also found in intermediate places, so in geology the life of a species or genus has not been interrupted. In other words, no group or species has come into existence twice." From these facts Mr. Wallace deduces the following important law:Every species has come into existence coincident both in space and time with a preexisting closely allied species." The adherents of development maintain that these facts, and many others of kindred significance, are only to be explained by the continuous operation of a great natural law of descent and divergence by which the present life of the earth has been derived from its preexisting life.

That the numberless forms of life should have been held as independently created, so long as the earth was regarded as having been recently and suddenly called into existence, was inevitable; but now, when it is known that the order of nature is extended backward into immeasurable time, the supposition that species were called into existence by hundreds of thousands of separate and special creations, running through the geological ages, and as we approach our own epoch suddenly and unaccountably ceasing, is held to be an unwarranted assumption which science can no longer accept. As remarked by the Rev. Baden Powell:The introduction of a new species is part of a series. But a series indicates a principle of regularity and law, as much in organic as in inorganic changes. The event is part of a regularly ordained mechanism of the evolution of the existing world out of former conditions, and as much subject to regular laws as any changes now taking place. If the series be regular, its subordinate links must each be so; the part cannot be less subject to law than the whole.

That species should be subject to exactly the same general laws of structure, growth, nutrition, and all other functions of organic life, and yet in the single instance of their mode of birth or origin should constitute exceptions to all physical law, is an incongruity so preposterous that no inductive mind can for a moment entertain it." This is the ground taken by the great majority of contemporary naturalists. They believe in evolution in some form as a great fact of nature; but many think that we know nothing as to how it has been brought about, while others hold that the problem of the modes and causes of evolution, although obscure, is no more barred from successful investigation than are the other phenomena of nature. - The following facts have been offered as throwing light upon the way in which the diversities of life have originated. Organic beings differ from inorganic in their modifiability. They are capable in various degrees of adaptation to new conditions. Plants taken from their native situations and cultivated in gardens undergo changes so great as often to render them no longer recognizable as the same plants.

The muscles are strengthened by exercise and the skin thickened and hardened by pressure, while the bones of men who put forth great physical exertion are more massive than the bones of those who do not labor. In the words of Mr. Spencer:There is in living organisms a margin of functional oscillations on all sides of a mean state, and a consequent margin of structural variation." These variations may become fixed through the law of hereditary descent. It is the law of transmission of characters which preserves species and varieties from generation to generation, oaks being always derived from oaks and dogs from ancestral dogs. It is not only the normal qualities that are perpetuated, but malformations, diseases, and individual peculiarities are also transmitted. While offspring tend to grow in the likeness of parents, they also tend to grow in unlikeness; while moulded upon the parental type, the resemblance is usually imperfect. Nor are variations confined to any particular organs or characters, but they may be manifested by every part, quality, or instinct of the creature. These divergences may be selected and fixed by breeding so as to give rise to new kinds or varieties. Nature begins the variation, art secures its perpetuation and increase.

How profound are the modifications that may be thus produced is shown in the numerous breeds of dogs, all of which belong to the same species. Not only have they reached extreme diversities in size (the largest being, according to Cuvier, 100 times larger than the smallest), but in muscular, bony, and nervous development, in form, strength, fleet-ness, and variety of instinct and intelligence, their divergences are almost equally remarkable. Domestic pigeons afford another example of the great plasticity of the living organism, by which it can be moulded into the extremest diversities. Naturalists believe that from a single species, the wild rock pigeon, there have arisen no fewer than 150 kinds that breed true or hold to the variety; and how deep have become the differences among them is thus stated by Prof. Huxley:In the first place, the back of the skull may differ a good deal, and the development of the bones of the face may vary a good deal; the beak varies a good deal; the shape of the lower jaw varies; the tongue varies very greatly, not only in correlation to the length and size of the beak, but it seems also to have a kind of independent variation of its own.

Then the amount of naked skin round the eyes and at the base of the beak may vary enormously; so may the length of the eyelids, the shape of the nostrils, and the length of the neck. I have already noticed the habit of blowing out the gullet, so remarkable in the pouter, and comparatively so in the others. There are great differences, too, in the size of the female and the male, the shape of the body, the number and width of the processes of the ribs, the development of the ribs, and the size, shape, and development of the breast bone. We may notice, too (and I mention the fact because it has been disputed by what is assumed to be high authority), the variation in number of the sacral vertebrae. The number of these varies from 11 to 14, and that without any diminution in the number of the vertebras of the back or of the tail. Then the number and position of the tail feathers may vary enormously, and so may the number of the primary and secondary feathers of the wings. Again, the length of the feet and of the beak, although they have no relation to each other, yet appear to go together; that is, you have a long beak wherever you have long feet.